Magnetism another “invisible” force

Magnetismanother “invisible” force Physics 100 Chapt 14

N-poles & S-poles S N Unlike poles attract N S

N-poles & S-poles S N like poles repel S N

N-poles & S-poles can never be isolated S N Break a magnet into two pieces: S N S N A new N-pole & S-pole are formed

Compass needles If left to its own accord, the N-pole of a magnet will try to point towards the North

The Earth is a magnet Earth’s geographic North pole is a Magnetic S-pole North Pole S N South Pole geographic South pole is a magnetic N-pole

Magnetic Fields B-field is in the direction of the net force Imagine a small N-pole is placed at the point x N Determine the magnetic force on the imaginary N-pole S N

B-field from a bar magnet S N

Rules S N B-field lines come out of N-poles & go into S-poles B-field lines never end Density of B-field strength of the field

E-field rules (reminder) E-field lines start on + charges and end on - charges Direction of E-field lines = direction of force on a + charge placed at that point (opposite for – charge) Where E-field lines are close together, the E field is large; where they are far apart, the E-field is small.

Horseshoe magnet

Earth’s B-field

Magnetarrays S N N N

Christian Oersted

Electric currents produce B-fields B I

Right-hand rule B

Current loop S N

Two current loops N S

Even more loops S N

Solenoid coil S N Looks like a bar magnet

Magnetic forces on electric currents I

Another right-hand rule I

Forces on two parallel wires I I Current in same direction: wires attract B

Forces on two parallel wires I Current in opposite directions: wires repel B I

Force law of Biot & Savart I1 I2 I1I2l d F = k l N A2 k=2x10-7 B d

Biot & Savart example I1I2l d 20A 20A F = k (20A)2 2m 0.01m N A2 F= 2x10-7 2m B F= 2 x 10-3N Small, but not tiny 0.01m

Electric motor F I I B F

Electric motor B I

Speakers Solenoid Electro-magnet Permanent magnet

Magnetism another “invisible” force